1. Field of the Invention
This invention relates to a weight detecting type sensor, and particularly to a wafer sensor conveniently usable to detect the arrival of a wafer at a desired position in a wafer conveying process.
2. Description of the Prior Art
Various apparatuses such as a coater for applying photoresist to wafers, an aligner for transferring circuit patterns to the wafers having the photoresist applied to them and a developer for developing the wafers having the circuit patterns transferred to them are used to manufacture semiconductor elements such as IC (integerated circuits) and LSI (large scale integrated circuits).
In these various apparatuses, wafers are placed one by one on a processing line, and a wafer cassette containing several tens of wafers is set at the beginning of the processing line so that the wafers are supplied one by one from this cassette to the processing line.
Additionally, a wafer cassette is also set at the terminating point of the processing line and is capable of containing several tens of wafers. For this reason, a wafer supply device and a wafer receiving device called a sender and a receiver, respectively, are disposed at the starting point and the terminating point of the processing line of each apparatus.
Further, a device called a buffer for temporarily containing wafers at an intermediate location on the wafer processing line and again placing them onto the processing line after a predetermined time is disposed at the intermediate location, and a wafer cassette capable of receiving several tens of wafers is also set at that location.
Now, in the sender, receiver and buffer as previously described, the wafer cassettes are adapted to be successively moved up or down after a wafer has been supplied onto the processing line or after the wafer has been received from the processing line. In each case, when a wafer has become positioned on a predetermined conveyor belt in the wafer cassette, the driving of the conveyor belt is stopped or started in response thereto.
Various types of wafer sensors have previously been used to detect whether a wafer rests on the conveyor belt. Typical examples of such wafer sensors are reflecting type (air reflecting type and light reflecting type) sensors and weight detecting type sensors.
The air reflecting type wafer sensor utilizes the phenomenon that air is blown against the back of a wafer and a turbulent flow is created when the air impinges on the wafer, but dust is blown up by the detecting air stream and contaminates the atmosphere around the wafer.
On the other hand, the light reflecting type wafer sensor utilizes the reflection of light incident on to the back of a wafer to detect whether the wafer rests on a conveyor belt at a predetermined position. However, sensors of this type have a disadvantage that the adjustment of sensitivity must be effected in conformity to the smoothness and surface-treated state of the reflecting surface of the wafer and operation is cumbersome.
The weight detecting type wafer sensor does not suffer from the disadvantages as described above. FIG. 1 of the accompanying drawings schematically shows a conventional weight detecting type wafer sensor.
FIG. 1A shows a state in which a wafer 1 has arrived at the vicinity of the sensor with the aid of a conveyor belt 2. The wafer 1 is thereafter conveyed to the left by the conveyor belt 2 as viewed in FIG. 1A and therefore, the leading end edge portion of the wafer rides onto a vane member 3 of the sensor which juts out upwardly. At this time the vane member 3 rotates about a pivot 4 as the wafer 1 advances, and as shown in FIG. 1B, the end portion of the wafer crosses a light transmitting type sensor 5. Thus, it is detected that the wafer 1 rests on the conveyor belt at a predetermined position. However, the wafer 1 is liable to be damaged by its sliding contact with the vane member 3 when it passes over the sensor. In addition, when the diameter and weight of the wafer vary, it is necessary to change the position of the pivot 4 of the vane member 3 or change the curvature of the contact portion of the vane member, and this has led to a disadvantage that adjustment is very cumbersome.